Device for evolution of oxygen with ternary electrocatalysts containing valve metals

ABSTRACT

A gas generating apparatus and method is described which utilizes a novel catalytic oxygen evolving electrode for such electrochemical systems as electrolysis cells and oxygen concentration cells. The electrochemical cells include a catalytic cathode and an improved catalytic anode positioned on opposite sides of, and in electrical contact with, a cation exchange membrane. A source of direct current potential between the cathode and the anode and means for removing gas from at least one of the electrodes are provided. The improved catalytic anode is a ternary platinum group reduced metal oxide alone or in combination with platinum group metals and/or platinum group metal oxides or mixtures of the foregoing having at least one valve metal component such as titanium, hafnium, zirconium, niobium, tantalum, and tungsten.

This application is a continuation of application Ser. No. 485,157,filed 4/15/83, now abandoned.

This invention relates to apparatus and methods for the generation andconcentration of gases, and more particularly, it relates to anapparatus and process for the generation and concentration of oxygen byelectrolysis using catalytic cathodes and improved catalytic anodes.

Electrochemical cells of the type utilizing an ion exchange membrane,otherwise known as and designated herein as a solid polymer electrolytemembrane, associated with a pair of catalytic electrodes have beendescribed in the prior art in various forms and applications. In oneform, such electrochemical cells may be utilized to generate electricalenergy and are commonly known as fuel cells. In another form, suchelectrochemical cells have been used in gas sensing and dosimeterdevices and processes. In still other forms, such electrochemical cellshave been utilized for gas generation utilizing specialelectrocatalysts.

Various metals and alloys are utilized as the catalytic electrodes forsuch fuel cells, for gas sensing and dosimeter cells and other cellssuch as those used for gas generation and gas concentrating. Theperformance of the catalyst at the gas evolving electrode (anode) or atthe gas concentrating electrode (anode) is crucial in the effectivenessand efficiency of the cell and consequently, it is crucial in theeconomics of the process. Such catalysts as platinum, platinum black andplatinum-alloys and mixtures thereof have been used in the past ascatalysts for these cells.

Electrochemical cells and methods for gas generation utilizing specificcatalysts and electrodes are well-known in the prior art. In U.S. Pat.No. 3,992,271, an electrolysis and oxygen concentrating cell utilizingan oxygen evolving catalytic anode is described. The oxygen evolvingcatalytic anode utilizes a reduced platinum-iridium oxide alloy andprovides improved performance and efficiency even though the use of suchreduced platinum-iridium oxide alloys as oxygen electrodes in fuel cellshad always resulted in poorer performance of the cell. However, theiridium, as well as the platinum, utilized as the alloying metals in thecatalyst of U.S. Pat. No. 3,992,271 is quite expensive, and it wasdiscovered in U.S. Pat. No. 4,039,409 that a reduced alloy oxide ofplatinum and ruthenium containing about 5 to 60% ruthenium, had betterperformance than the reduced platinum-iridium oxide alloy catalyst, andit was much less expensive to produce since the cost of ruthenium saltsutilized to fabricate the platinum-ruthenium alloy was approximatelyhalf the cost of the iridium salts. Thus, in U.S. Pat. No. 4,039,409,there was provided a catalytic cathode and a gas evolving catalyticanode containing a reduced platinum-ruthenium alloy oxide containing 5to 60% by weight of ruthenium positioned on opposite faces of a cationexchange membrane.

Although the prior art gas generating and concentrating devices andprocesses work efficiently, it is always desirable to improve theefficiency of the devices and processes and at the same time reduce thecost of the devices and processes. Furthermore, it is always desirableto improve the resistance of the materials used in the electrochemicalcells to the corrosion effects of chemicals. Since these electrochemicalgas generating and gas concentrating devices utilize and/or generateacid media, there is a tendency of these acids (hydrogen ions or protonsand other acid media) to corrode the catalysts, especially the catalystsused at the anode and thereby reduce the life of the device.

Stabilized electrocatalysts have been used as electrodes in processesand devices for the generation of chlorine by electrolysis of an aqueousalkali metal halide at the anode of an electrolysis cell which includesa solid polymer electrolyte in the form of a cation exchange membrane toseparate the cell into catholyte and anolyte chambers. The catalyticelectrodes at which the chlorine and caustic are produced are thin,porous, gas permeable catalytic electrodes which are bonded to andembedded in opposite surfaces of the membrane so that the chlorine isgenerated at the electrode membrane interface. This results inelectrodes which have very low overvoltages for chlorine discharge andthe production of caustic. The catalytic electrodes recommended for thiselectrolysis cell for the production of chlorine include a catalyticmaterial comprising at least one reduced platinum group metal oxidewhich is thermally stabilized by heating reduced oxides in the presenceof oxygen, and in a preferred embodiment, the electrodes arefluorocarbon bonded with thermally stabilized reduced oxides of aplatinum group metal such as platinum, palladium, iridium, rhodium,ruthenium, and osmium. In the electrolysis cell for the production ofchlorine, it was also discovered that one or more reduced oxides of avalve metal such as titanium, tantalum, niobium, zirconium, hafnium,vanadium, or tungsten could be added to stabilize the electrode againstoxygen, chlorine and the generally harsh electrolysis conditions. Theforegoing disclosure relates only to an electrochemical cell for theproduction of chlorine from a medium such as aqueous sodium chloride.

The choice of catalyst in an electrochemical cell and its effectivenessin a given cell, depends upon a complex set of variables such as thesurface area of a catalyst, availability of oxides of its species on thecatalyst surface, contaminants in the reactant, and the nature of theconversion taking place in the cell. Consequently, it is and always hasbeen difficult to predict the applicability of a catalyst useful in oneelectrochemical cell system to a different system. Even though one typeof catalyst may produce advantageous results in one type ofelectrochemical cell system, it does not always follow that such animprovement will be realized when the same catalyst is utilized in adifferent electrochemical cell system. As explained above, it is alwaysdesirable not only to improve the stability of the catalysts and otherelements of the electrochemical cells, it is always desirable to improvethe efficiencies of the electrochemical cells and the processes carriedout therein.

It is, therefore, the primary object of the present invention to providean improved method and apparatus for gas generation utilizing animproved electrocatalyst at the gas evolving electrode.

It is a further object of the present invention to provide an improvedgas generation apparatus and process with which to concentrate gases orproduce gases by electrolysis which utilizes a catalyst which providesimproved performance, improved stability and reduced cost.

Another object of this invention is to provide a method and apparatusfor producing oxygen by the electrolysis of media which evolve oxygenand protons by electrolysis, with substantially lower and improved cellvoltages.

Still another object of the present invention is to provide an improvedgas generation apparatus and a process for concentrating oxygen orproducing oxygen by electrolysis utilizing an improved, stable catalystat the oxygen evolving anode.

Other objects and advantages of the invention will become apparent fromthe following description.

In accordance with the invention, oxygen is electrolytically generatedby the steps comprising, providing a catalytic cathode; providing acatalytic oxygen evolving anode; positioning a cation exchange membranebetween, and in electrical contact with, the cathode and the anode;providing a direct potential between the cathode and the anode andsupplying water to one of the electrodes to be acted onelectrochemically to evolve oxygen at the anode, wherein the improvementcomprises the step of providing a catalyst at the oxygen evolving anodeconsisting of at least two platinum group metal-containing compounds andat least one valve metal-containing compound. In another aspect of theinvention, there is provided an apparatus for the production of oxygenby the electrolysis of a medium which evolves oxygen and protons byelectrolysis, comprising, a catalytic cathode; a catalytic oxygenevolving anode comprising a catalyst consisting of at least two platinumgroup metal-containing compounds and at least one valve metal-containingcompound; an ion exchange membrane (solid polymer electrolyte membrane)disposed between and in electrical contact with, the cathode and theanode; means for providing a direct potential between the cathode andthe anode; and means for supplying a medium which evolves oxygen andprotons by electrolysis.

In another aspect of the invention, there is provided a process for theproduction of oxygen by the electrolysis of media which evolve oxygenand protons by electrolysis, comprising;

(a) continuously supplying the medium which evolves oxygen and protons,to a catalytic oxygen evolving anode in an electrolytic cell wherein thecatalytic oxygen evolving anode is separated from a catalytic cathode bya cation exchange membrane, and the catalytic oxygen evolving anode andthe catalytic cathode are in electrical contact with the cation exchangemembrane, the catalytic oxygen evolving anode comprising a catalystconsisting of at least two platinum group metal-containing compounds andat least one valve metal-containing compound;

(b) continuously providing an aqueous medium to the catalytic cathode;

(c) supplying current to the anode and the cathode to electrolyze themedium which evolves oxygen and protons at the anode; and,

(d) removing oxygen from the anode and gas formed from the protons fromthe cathode.

In another aspect of the invention, there is provided a method forelectrolytically generating oxygen from oxygen contained in a gaseousstream by concentrating the oxygen at the anode, comprising the stepsof:

(a) providing a catalytic cathode;

(b) providing a catalytic anode;

(c) positioning an ion exchange membrane between, and in electricalcontact with, the cathode and the anode;

(d) providing a direct current potential between the cathode and theanode;

(e) providing a gaseous stream including oxygen at the cathode; and,

(f) supplying water to one of the electrodes to be acted onelectrochemically;

the improvement comprising providing a catalyst at the oxygenconcentrating anode consisting of at least two platinum groupmetal-containing compounds and at least one valve metal-containingcompound.

Critical in the process and apparatus of the present invention is theuse of a catalyst at the catalytic anode wherein the catalyst consistsof at least two platinum group metal-containing compounds and at leastone valve metal-containing compound. As used herein, the valve metalsare titanium, tantalum, niobium, zirconium, hafnium, vanadium andtungsten. The platinum group metals are platinum, palladium, rhodium,ruthenium, osmium, and iridium. In this invention, "metal-containingcompounds" is defined as the metal oxides, the reduced metal oxides, themetals and mixtures and/or alloys thereof.

These and various other objects, features and advantages of theinvention can be best understood from the following description taken inconnection with the accompanying drawings in which:

FIG. 1 is a sectional view of a gas generation apparatus for theproduction of gases, such as oxygen and hydrogen, by electrolysis whichis capable of carrying out the process of the invention and which isconstructed with the catalytic anode of the present invention; and,

FIG. 2 is a sectional view of a gas generating apparatus of the gasconcentrator type or air depolarizer electrolyzer having a catalyticanode made in accordance with the invention and capable of carrying outthe process of the invention.

The devices illustrated in FIGS. 1 and 2 are used to illustrate thecatalytic anodes of the present invention and are clearly described inU.S. Pat. No. 4,039,409.

In FIG. 1 of the drawings, there is shown a gas generation apparatus inthe form of an electrolysis cell. In this cell, oxygen anode chamber 22(anolyte chamber) communicates with oxygen outlet 25 while hydrogencathode chamber 18 (catholyte chamber) communicates with hydrogen outlet26. In the operation of the electrolysis cell of FIG. 1, a directcurrent potential is applied across catalytic anode 11 and catalyticcathode 12 from batteries 16 while a hydrogen-containing compound or amedium which evolves oxygen and protons, such as water, aqueous sulfuricacid, aqueous sodium sulfate, and the like, is supplied at the catalyticanode. The direct current applied across catalytic anode 11 andcatalytic cathode 12 dissociates the medium at the anode to produceoxygen and protons (hydrogen ions). The hydrogen accumulates incatholyte chamber 18 and is removed through outlet 26 while oxygenaccumulates in anolyte chamber 22 and is removed through outlet 25.

In the electrolysis cell of FIG. 1, no gases are supplied to theapparatus but a direct current potential is applied across the improvedcatalytic anode of the present invention to the catalytic cathode fromthe battery, and a hydrogen-containing or proton-containing compoundsuch as water, for example, is supplied at the improved catalytic anode.The catalytic anode is a catalyst consisting of at least two platinumgroup metal-containing compounds and at least one valve metal-containingcompound. Dissociation of the hydrogen-containing or proton-containingcompound at the catalytic anode results in molecular hydrogen gas (H₂)being produced at the catalytic cathode while oxygen gas is produced atthe improved catalytic anode. With a cation exchange membrane, thereactions at the electrodes are as follows:

At the improved catalytic anode:

    H.sub.2 O→1/2O.sub.2 +2H.sup.+ +2e.sup.-

At the catalytic cathode:

    2H.sup.+ +2e.sup.- →H.sub.2

In the electrolysis cell described above, the medium which evolvesoxygen and protons, that is the hydrogen-containing or proton-containingcompounds which also contain oxygen, for example, water, required toproduce hydrogen ions (protons) and oxygen, for example, by dissociationat the improved catalytic anode, is supplied through the catalytic anodeby flooding the anode chamber (anolyte chamber) or through the use ofwicking. This mode of supply of water or other media which evolve oxygenand protons, or oxygen and hydrogen ions, is preferable to a cathodewater feed.

Briefly, in FIG. 2, there is shown generally at 10 a gas generationapparatus in the form of an oxygen concentrator embodying the inventionand capable of carrying out the process of the invention. The samenumbers have been used to show similar parts in FIGS. 1 and 2. Apparatus10 is shown with a catalytic anode 11 which consists of at least twoplatinum group metal-containing compounds and at least one valvemetal-containing compound, a catalytic cathode 12 and an ion exchangemembrane or solid polymer electrolyte membrane 13 positioned orinterposed between, and in electrical contact with anode 11 and cathode12. Electrical leads 14 and 15 are connected to electrodes 11 and 12 andto an external power source 16 shown in the form of a battery connectedacross the electrodes.

An oxidant, such as air or impure oxygen, is supplied to cathode 12through an inlet conduit 17 and chamber 18 (the catholyte chamber)formed by endplate 19, gasket 20 and cathode 12. A valved outlet 21 isprovided for exhaust impurities from cathode chamber 18. An output gaschamber 22 (anode chamber) is formed by anode 11, endplate 23 and gasket24. The concentrated oxygen provided to anode chamber 22 is supplied toa suitable outlet conduit 25 for consumption or storage.

The gas concentrator shown in FIG. 2 operates by supplying the gas (airor impure oxygen, both of which are free of carbon monoxide or carbonfuel contaminants) through inlet 17 and cathode chamber 18 to catalyticcathode 12. A direct current potential is applied across catalyticcathode 12 and catalytic anode 11 from battery 16 to concentrate the gas(oxygen) which collects in anode chamber 22 and is removed throughoutlet 25. When the gas (air or impure oxygen) is furnished through thecathode, water either from a wicking device, from steam, or from someother source of humidification, or through back diffusion through theion exchange membrane is furnished to the cathode. With a cationexchange membrane employed between the anode and the cathode, hydrogenions are conducted through the membrane from the oxygen output side tothe oxygen input side. Water, which is formed at the oxygen input side,migrates through the electrolyte from the oxygen output side to theoxygen input side with the hydrogen ion. Since water does not backdiffuse rapidly enough from the oxygen input side to the oxgyen outputside to replenish water at the catalytic anode, additional water isrequired because water is either dissociated or migrates with hydrogenions. This is most easily accomplished by supplying water to the gas(oxygen) output side by flooding the anode chamber or through use ofwicking means. The reactions for the gas (oxygen) concentration at thevarious electrodes are as follows:

at the cathode;

    1/2O.sub.2 +2H.sup.+ →H.sub.2 O

at the anode;

    H.sub.2 O→1/2O.sub.2 +2H.sup.+ +2e.sup.-

Although an anion exchange membrane may be utilized as the solid polymerelectrolyte for oxygen concentration, there is an advantage in utilizinga cation exchange membrane for the solid polymer electrolyte in thatcarbon dioxide pick-up from the air is minimized.

It has been found that an improved gas generation apparatus and animproved process for generating gas from a medium which evolves oxygenand protons by electrolysis, for example, from water, is possible byutilizing an improved catalytic anode which provides superiorperformance and superior stability in oxygen concentration and in thegeneration of oxygen and hydrogen by electrolysis. It has been foundthat catalysts consisting of at least two platinum groupmetal-containing compounds and at least one valve metal-containingcompound result in improved performance in that the voltage required inthe operation of the electrolysis cell may be less thereby reducingpower consumption during usage, and in improved stability in that thecatalytic anode made in accordance with the present invention increasesthe life of the electrolysis cell also thereby resulting in increasedeconomy. Furthermore, the use of less-costly platinum groupmetal-containing compounds in conjunction with a less-costly valve metalcontaining compound results in a lower initial cost for the electrolysiscell.

As explained above, it has been discovered that the improved catalyticanode having a catalyst consisting of at least two platinum groupmetal-containing compounds and at least one valve metal-containingcompound may be used in electrolysis cells for the concentration ofoxygen from air or certain other oxygen-containing gases or for thepreparation of oxygen by the electrolysis of a medium which evolvesoxygen and protons (hydrogen). Thus, oxygen and hydrogen can be obtainedfrom water, aqueous sulfuric acid, aqueous sodium sulfate and the like.The medium which evolves oxygen and protons also includes deuteriumoxide and tritium oxide. For example, if water is supplied to one orboth of the electrodes of the electrolysis cell, dissociation, that is,electrolysis of the water, can take place, and oxygen and hydrogen areproduced at the two electrodes. Hydrogen ion is selectively transportedacross the ion exchange membrane. When the apparatus operates in the gasconcentration mode, such as the concentration of oxygen, for example,the driving force of the applied potential as well as the permselectivenature of the ion exchange membrane (solid polymer electrolyte) permitsan oxidant such as air or impure oxygen to be fed to the cathode of theapparatus. Hydrogen ion is formed at one of the catalytic electrodes(the anode) and passes through the solid polymer electrolyte membrane tothe opposite electrode (the cathode) where water is formed.

A variety of ion exchange membranes may be used in the cell. One whichfunctions very adequately is a perfluorocarbon sulfonic acid solidpolymer electrolyte sold by E. I. Dupont de Nemours & Co. under itstrade designation NAFION. Various catalytic materials such as platinumblack, for example, may be utilized for the catalytic cathode. Thecatalytic electrodes, both cathode and anode, are customarily pressedinto, embedded upon or mounted directly upon the surface of the ionexchange membrane otherwise designated in the prior art as the solidpolymer electrolyte. The catalytic cathode and the solid polymerelectrolyte membrane can be chosen by one skilled in the art and are notcritical in the practice of the present invention. Various catalyticcathode materials, alternative ion exchange membrane materials, theirproperties and mode of preparation and the like, are described in theprior art including U.S. Pat. No. 3,297,484. The catalytic electrodesare generally of the thin, porous, gas permeable type which are bondedto and embedded in opposite surfaces of the membrane so that the gasesare generated right at the electrode-membrane interface.

In its broadest aspect, the improvement of the present invention isdirected to the catalytic anode or catalytic oxygen-evolving anode orcatalytic oxygen concentrating anode wherein the improvement comprises acatalyst consisting of at least two platinum group metal-containingcompounds and at least one valve metal-containing compound. In the mostpreferred embodiment, the improved anode comprises a ternary catalystconsisting of two platinum group metal-containing compounds and a valvemetal-containing compound. By use of the phrase "platinum groupmetal-containing compound" as used herein, is meant a reduced platinumgroup metal oxide, a reduced platinum group metal oxide in combinationwith a platinum group metal, a reduced platinum group metal oxide incombination with a platinum group metal oxide, or a reduced platinumgroup metal oxide in combination with platinum group metals and platinumgroup metal oxides, and mixtures thereof. The phrase also embracesalloys of the foregoing and alloys containing platinum group metals.Examples of using platinum group metals are platinum, palladium,rhodium, ruthenium, osmium and iridium. Although the catalytic anodes ofthe present invention generally embrace the platinum group metals andmetal oxides, the preferred catalytic anodes are made from the reducedmetal oxides, such as reduced ruthenium oxide, reduced iridium oxide,reduced platinum oxide, reduced palladium oxide, reduced rhodium oxideand reduced osmium oxide.

In accordance with the present invention, it has been found that atleast two platinum group metal-containing compounds must be present inthe improved catalytic anode. Mixtures or alloys of the platinum groupmetal-containing compounds have been found to be more stable, and anycombination of the platinum group metal-containing compounds may be usedin mixtures or alloys which make up the composition of the catalyticanode. For example, the catalytic anode may comprise a reduced platinumgroup metal oxide and a platinum metal; it may comprise a platinum groupmetal oxide and a reduced platinum group metal oxide; it may comprise aplatinum group metal, a platinum group metal oxide and a reducedplatinum group metal oxide; it may comprise two or more reduced platinumgroup metal oxides with two or more platinum group metal oxides and thelike.

The improved catalytic anode must also comprise at least one valvemetal-containing compound. As used herein, valve metal-containingcompound is defined as a valve metal oxide, a reduced valve metal oxide,valve metal and mixtures thereof. The term also embraces alloys of theforegoing valve metals and alloys containing valve metals. The valvemetals include titanium, tantalum, niobium, zirconium, hafnium,vanadium, and tungsten. In the preferred embodimemts of the presentinvention one or more reduced oxides of the valve metals such as thereduced oxide of titanium, the reduced oxide or tantalum, the reducedoxide of niobium, the reduced oxide of zirconium, the reduced oxide ofhafnium, the reduced oxide of vanadium, and the reduced oxide oftungsten, may be used to stabilize the catalytic anode against attack byacid, i.e., when protons are present in the medium being electrolyzed,the medium is acidic and generally such acidic media attack theelectrode and reduce the life of the electrode. It has been found thatthe electrodes made in accordance with the present invention andcontaining at least one valve-metal containing compound and at least twoplatinum group-metal containing compounds substantially extend the lifeof the catalytic anode. The improved catalytic anodes of the inventionmay also contain various mixtures and alloys of the valvemetal-containing compounds. For example, the improved catalytic anodesmay comprise one or more reduced oxides of a valve metal, one or moreoxides of a valve metal or one or more valve metals or any mixture ofthe foregoing. For example, the improved catalytic anode may comprise(along with at least two platinum group metal-containing compounds asdescribed above) a reduced oxide of titanium and tantalum metal, or anoxide of hafnium and a reduced oxide of niobium or titanium metal andtantalum metal or any combination in the form of mixtures and/or alloysof the valve metal-containing compounds as defined above.

Examples of preferred improved catalytic anodes of the invention areternary alloys of 50% platinum-25% ruthenium-25% titanium, 50%platinum-25% ruthenium-25% hafnium, 50% ruthenium-25% iridium-25%titanium, 50% ruthenium-25% iridium-25% hafnium, 50% ruthenium-25%iridium-25% niobium, 50% ruthenium-25% iridium-25% tungsten, 50%ruthenium-25% iridium-25% tantalum, and 50% ruthenium-25% iridium-25%zirconium. The foregoing preferred ternary alloys are the reduced metaloxides.

The amount or concentration of each of the platinum groupmetal-containing compounds and the valve metal-containing compounds inthe mixture or alloy composition of the catalytic anode is not criticalas long as the alloy contains at least two platinum groupmetal-containing compounds and at least one valve metal-containingcompound. Generally, the at least two platinum group metal-containingcompounds comprise at least 50% by weight of the alloy or mixtures ofthe metal-containing compounds. Up to 50% by weight of the valvemetal-containing compound is useful with the preferred amount of valvemetal-containing compound being about 0.5%-50%, and the most preferredamount of the valve metal-containing material being about 25%-50% byweight. Thus, when the concentration of the valve metal-containingcompound is 0.5% by weight, the composition of platinum groupmetal-containing compounds comprises 99.5% of at least two platinumgroup metal containing compounds.

The platinum group metal-containing compounds of the catalystcomposition for the catalytic anode may be present in equal amounts ormay be present in any suitable combination. For example, about 99.5% ofthe platinum group metal-containing compounds may be a first platinumgroup metal-containing compound, and the second platinum groupmetal-containing compound, may be 0.5% by weight. In preferredembodiments, the second platinum group metal-containing compound of thecomposition is about one-half the amount of the first platinum groupmetal-containing compound in the composition. Of the entire compositionincluding the platinum group metal-containing compounds and the valvemetal-containing compounds, the second platinum group metal-containingcompound generally contains up to about 25% by weight of the compositionand preferably from about 5 to about 25% by weight of the composition.Unless otherwise indicated, all percentages are weight percent.

Other materials may also be included in the catalyst composition of theimproved catalytic anode as long as the materials do not affect theperformance or the stability of the electrode in the processes andapparatus of the invention. For example, various binders and extenderswhich are well-known in the art may be used in the catalytic anode.Extenders are generally materials having good conductivity and maycontribute to the stability, life, porosity, conductivity and the likeof the catalyst material. One such conductive extender has been found tobe graphite and may be used in an amount up to 30% by weight of thecomposition. In other cases, it has been found advantageous to use abinder to bond the catalyst materials, that is the alloys or mixtures,such as the ternary alloy, to the solid polymer electrolyte membrane.Binders are well-known in the art and include polytetrafluoroethyleneparticles which may be mixed with a mixture or alloy of the at least twoplatinum group metal-containing compounds and the at least one valvemetal-containing compound prior to fixing the material to the solidpolymer electrolyte membrane or prior to casting the catalyticelectrode, whichever technique is used for mounting the electrode to thesolid polymer electrolyte membrane.

The mixtures and alloys of the present invention may be made in anymanner well-known in the art. In one preferred method known as theAdam's method, the catalytic alloy can be prepared by thermallydecomposing the mixed metal salts of the compounds used in the alloy inthe presence of a strong oxidant such as sodium nitrate (NaNO₃),followed by subsequent electrochemical reduction. For example, by theAdam's method as described in U.S. Pat. No. 4,039,409, the chloridesalts of ruthenium, iridium and tantalum are mixed with an excess sodiumnitrate. Ruthenium chloride, iridium chloride and tantalum chloride infinely-divided form are mixed in the same weight ratio as desired in thefinal alloy with the excess sodium nitrate, and the mixture is fused ina silica dish at about 500° C. for 3 hours. The residue is then washedthoroughly to remove any water-soluble salts such as soluble nitratesand chlorides leaving a residue of the ruthenium oxide-iridium oxide,tantalum oxide. The resulting suspension of mixed oxides is reduced byan electrochemical reduction technique, and the product is a reducedruthenium-iridium-tantalum alloy. The alloy may be dried thoroughly,comminuted and then graded as desired, for example, by use of sievessuch as 400 mesh nylon screen. Stabilization is then affected bytemperature (thermal) stabilization, i.e., by heating the platinum groupmetal-containing compound at a temperature below that at which it beginsto decompose, and preferably, by heating the reduced oxides of theplatinum group metal at a temperature at which the reduced oxides beginto decompose. Thus, the reduced oxides of the platinum group metals maybe heated at about 350°-750° C. from 30 minutes to 6 hours. Thepreferred thermal stabilization procedure is accomplished by heating thereduced oxides for 1 hour at temperatures in the range of 550° to 600°C. The materials are further stabilized by mixing them with otherreduced oxides of other platinum group metals and also with the reducedoxides of the valve metals. In one mode of the present invention, it hasbeen found that the ternary alloys of reduced oxides of the platinumgroup metals are very effective in producing stable, long-lived anodesin the oxygen gas generation processes and apparatus of the presentinvention. In the case of the ternary alloy, the composition ispreferably 5% to about 25% by weight of reduced oxides of one platinumgroup metal-containing compound, approximately 50% by weight-of anotherreduced oxide of a platinum group metal-containing compound and theremainder a valve metal-containing compound. The catalyst compositionsmade in accordance with the present invention for use in the oxygengenerating or concentration apparatus may be sieved through theappropriate size mesh screen where desired.

Anode and cathode current collectors well-known in the art may also beused to engage the catalytic anode and the catalytic cathoderespectively to make electrical contact therewith.

EXAMPLE 1

To illustrate the operational characteristics of an electrolysis cellutilizing the improved catalytic anodes of the invention and to show thesuperior performance of the oxygen generating processes and apparatus ofthe present invention, electrolysis cells similar to those in FIG. 1were constructed and various oxygen evolving anodes were used therein.The catalyst cathode of each of the electrolysis cells contained aplatinum catalyst, such as platinum black. The solid polymer electrolytemembrane was a cation exchange membrane having electrodes with activeareas of approximately 1/20 ft² operating in a flooded anode mode. Thecells were operated at 150° F. The performance of several of theelectrolysis cells were compared when the improved catalytic anodeshaving at least two platinum group metal-containing compounds and atleast one valve metal-containing compound, and more specificallycontaining a ternary alloy, were compared with prior art catalyticanodes having reduced platinum group metal oxides. In one case a 50%platinum-50% iridium (reduced metal oxide) was used as the anodecatalyst, and in another case the prior art reduced oxide of 80%platinum-20% ruthenium was used as the anode catalyst material. Theperformance designated as cell potential in volts for various currentdensities in amperes per square foot are set forth in Table 1 for thevarious alloys and mixtures shown in the table.

                                      TABLE 1                                     __________________________________________________________________________    PERFORMANCE COMPARISON OF ELECTROLYSIS CELLS USING BINARY                     AND TERNARY ALLOY CATALYST ANODES                                             CELL POTENTIAL (VOLTS)                                                        Current                                                                       Density                                                                             *Pt(50)/Ir(50)                                                                        *Pt(80)/Ru(20)                                                                        *Pt(50)/Ru(25)/Hf(25)                                                                    *Pt(50)/Ru(25)/Ta(25)                        (amps/ft.sup.2)                                                                     Anode Catalyst                                                                        Anode Catalyst                                                                        Anode Catalyst                                                                           Anode Catalyst                               __________________________________________________________________________    100   1.53    1.51    1.50       1.54                                         200   1.59    1.57    1.57       1.59                                         300   1.63    1.62    1.62       1.64                                         __________________________________________________________________________     *Designates percentage of reduced metal oxide alloy in anode catalyst.   

The cell was operated at about 66° C. It can be seen from the data inTable 1 that the results achieved with the electrolysis cell containinga 50% platinum/25% ruthenium/25% hafnium ternary catalyst anodedemonstrates the feasibility of lower cost, high performance catalystsof the present invention for electrolysis. In certain instances, thecell voltage using the less expensive ternary catalyst of the presentinvention is better than the voltage in those cells using the prior artbinary (two metal-containing) catalysts. For example, the improved anodecatatlyst of the invention made from the reduced oxides of 50%platinum/25% ruthenium/25% hafnium show improved performance (less cellvoltage or reduced cell voltage) at a current density of 100 amps/ft.²and 300 amps/ft.².

EXAMPLE 2

Several reduced metal oxides containing ruthenium and iridium platinumgroup metals were formed into alloys and/or mixtures with the valvemetals. The modified Adams method discussed above was used to preparethe ternary alloys and/or mixtures from the mixed halides or nitratesalts of the various metals fused with sodium nitrate to form theoxides. These oxides were then electrochemically reduced, and theresulting materials (alloy and/or mixtures) were formed into electrodesand placed upon the surface of a solid polymer electrolyte membraneidentified as NAFION and disclosed in detail above. A reduced platinumblack catalyst was used as the cathode material. The hydrated membranehaving the defined catalytic anode and cathode was placed in anelectrolysis cell similar to that shown in FIG. 1 and described in thespecification. The performance of the cell at about 82° C. (180° F.) isshown in Table 2 below where several ternary catalyst anodes of thepresent invention containing the platinum group metals, ruthenium andiridium, and various valve metals, are compared with a prior artplatinum/iridium catalyst anode.

                  TABLE 2                                                         ______________________________________                                        PERFORMANCE COMPARISON OF                                                     ELECTROLYSIS CELLS USING BINARY                                               AND TERNARY ALLOY CATALYST ANODES                                             CELL POTENTIAL (VOLTS)                                                        Current             *Ru(50)/Ir(25)/                                                                            *Ru(50)/Ir(25)/                              Density *Pt(50)/Ir(50)                                                                            Ta(25)       Zr(25)                                       (amps/ft.sup.2)                                                                       Anode Catalyst                                                                            Anode Catalyst                                                                             Anode Catalyst                               ______________________________________                                         100    1.475       1.438        1.447                                         500    1.649       1.609        1.617                                        1000    1.805       1.772        1.774                                        1500    1.941       1.914        1.906                                        ______________________________________                                         *Designates percentage of reduced metal oxide alloy in anode catalyst.   

EXAMPLE 3

Valve metal-containing ruthenium/iridium catalysts were made inaccordance with the procedure set forth in Example 2, and solid polymerelectrolyte membranes using the described ternary catalysts as anodesand platinum black as cathodes were used in electrolysis cells identicalto those described in Example 2. The test cells were run at about 82° C.(180° F.). Electrolysis cell voltage at various current densities forthese electrolysis cells using the improved electrode (anodes) of theinvention are shown in Table 3 below. The cell voltages may be comparedwith the prior art 50% platinum/50% iridium reduced oxide anode catalystshown in Table 2.

                                      TABLE 3                                     __________________________________________________________________________    CELL POTENTIALS AT VARIOUS CURRENT DENSITIES OF ELECTROLYSIS CELLS USING      TERNARY ALLOY CATALYST ANODES                                                 CELL POTENTIAL (d.c. VOLTS)                                                   Current                                                                             *Ru(50)/Ir(25)/Ti(25)                                                                    *Ru(50)/Ir(25)/Hf(25)                                                                    *Ru(50)/Ir(25)/Nb(25)                                                                     *Ru(50)/Ir(25)/W(25)                  Desity                                                                              ANODE      ANODE      ANODE       ANODE                                 (amps/ft.sup.2)                                                                     CATALYST   CATALYST   CATALYST    CATALYST                              __________________________________________________________________________     100  1.494      1.447      1.471       1.449                                  500  1.666      1.627      1.650       1.613                                 1000  1.825      1.780      1.812       1.758                                 1500  1.956      1.860      1.945       1.892                                 __________________________________________________________________________     *Designates percentage of reduced metal oxide alloy in anode catalyst.   

The foregoing data demonstrates that less expensive oxygen generatinganodes can be prepared and operated in electrolysis cells as efficientlyand in many cases more efficiently than the prior art catalytic oxygenevolving anodes. Power reduction, that is, reduced cell voltages, issignificant, and if the reduction in cell potential is even as little as50 millivolts, substantial savings in power consumption can be realized.The foregoing data demonstrates efficient oxygen evolution with theimproved ternary catalyst anode of the invention.

While other modifications of the invention and variations thereof whichmay be employed within the scope of the invention have not beendescribed, the invention is intended to include such modifications asmay be embraced within the following claims. What is claimed is:

1. Apparatus for the evolution of oxygen and protons by electrolysis ofan oxygen containing compound whereby gaseous oxygen and gaseoushydrogen are produced in said apparatus comprising:a catalytic cathode;a catalytic oxygen evolving anode comprising a ternary catalyst of afirst platinum group metal compound, a second platinum group metalcompound and a valve metal containing compound, wherein said valve metalcontaining compound comprises about 1/2 percent to about 50 percent byweight, the first platinum containing compound comprises about 5 percentto about 25 percent by weight, and the second platinum group metalcompound comprises the remainder of said ternary catalyst; an ionexchange membrane for dividing the apparatus into anode and cathodechambers, said membrane being disposed between and in electrical contactwith the catalytic cathode and catalytic oxygen evolving anode, thecathode and anode being bonded to the membrane; means for providing adirect current potential to said anode whereby an anodic potential isapplied to the anode containing the oxygen evolving ternary catalyst;means for continuously introducing an oxygen evolving compound to theanode chamber to evolve gaseous oxygen and protons in an electrolysisreaction, and means for providing a direct current potential to thecathode whereby a cathodic potential is applied to the cathode therebycausing transport of said protons across said membrane to evolve gaseoushydrogen at the cathode.
 2. The apparatus of claim 1 wherein thecatalyst of the oxygen evolving anode comprises about 50 percent byweight ruthenium, about 25 percent by weight iridium, and about 25percent by weight of tantalum.